Re: ATM forwarded msg on collimation

[Bob Rubendunst <softm@softm.com>]

> Last month I took the time to tweek my collimation by the trusted
> method of sight tube, cheshire and auto collimator in succession to
> arrive at a very good collimation. However, when I'd use the laser
> collimation, it would inevitably tell me that my collimation was out
> by several inches even though the cheshire/auto collimator solution
> suggested that I was right on the numbers. Incidentally, the laser
> has about 1/4 inch run-out in my Tectron low profile focuser.

I have had the same experience. I believe it is because the laser collimator is only 
useful if the focuser and secondary mirror assemblies are already absolutely perfectly 
aligned. (That is, the focuser is square to the tube, the secondary is centered & offset 
in the tube & rotated to be square with the focuser, and that the primary mirror's 
optical axis is centered in the tube.)

In addition, if there is any slop in the focuser (e.g. a telescoping focuser tube on a 
Coulter 8" Dob), a laser is a bad alignment tool.

The cheshire works better because even when the focuser or secondary are poorly aligned, 
you can still get a decent "compromise" adjustment. What you see in the Cheshire has a 
lot more to do with how the scope performs compared to the position of a laser dot. Using 
the Cheshire requires centering images, but with the laser, your'e only centering a dot, 
and assuming that means everything is centered.

A useful exercise is purposely misaligning the elements in your scope and seeing how it 
performs.





-Bob Rubendunst
Soft Machines
http://www.softm.com